Isn’t there a real risk point in the latter part of this decade when presumably 50+ year old stock will further degrade in performance and other options such as single DMUs will also be approaching end of life ?
No.
The HST sets are mechanically the youngest units in the fleet. The power cars in particular were rebuilt with the MTU mods around 2005. The modular design of DMUs means they also cycle out mechanical components on a very regular basis, so you're only really talking about an aged body shell - and this can be patched (or run round with visible corrosion holes in it, depending on the will of the operator...). I suspect (though, no doubt others can confirm) that the main HST issues stem from a refurbishment/door project that was specified and budgeted as a short term solution, rather than a major investment.
Would seem a high possibility of a perfect storm.
Pretty unlikely. Actually, in most cases the opposite is true. Look at the issues at TfW and WMT (and by extension, EMR), where the fleet plan is based on wholesale replacement and introduction of a new fleet. Teething issues are far more likely to throw an almighty spanner in the works. The question with the HSTs, for me anyway, is are the current problems essentially on-going teething issues?